Coated photographic products normally comprise one or more layers of a hydrophilic colloidal composition. The vehicle for these compositions is usually gelatin and the layers are coated onto substrates including paper and acetate and polyester films. The substrates may possess a thin subbing layer to promote adhesion of the layers. The subbing layer is typically a hydrophilic colloidal composition comprising gelatin and other addenda including a cross-linking agent, other polymers, matte particles, and surfactants.
Sometimes multilayer photographic products are coated in four or more stages that may follow in one continuous manufacturing line. The second and later stages are coated onto product layers that have been coated and dried; only the first stage is coated directly on an original receiving surface of paper or plastic that may be subbed. The product layers are typically dispersions and emulsions in a gelatin vehicle. The compositions typically contain surfactants as dispersing aids. The compositions are chemically complex.
Economical and reliable manufacturing requires high speeds of coating and a robust coating process. Indeed, it is desirable that manufacturing processes be so reliable that the numerous inspection, sorting, disposition, rework steps, and large inventories characteristic of past manufacturing processes be eliminated to reduce cost. At the same time, it is desirable to reduce greatly the time it takes to formulate new products and bring them to market. Reduced cycle time, as it is called, can make the difference between success and failure in the highly competitive global marketplace. As coating speeds have increased for economic reasons, significant differences in coating latitude among receiving surfaces have become apparent. Manufacturing photographic products may involve several coating operations with as many different receiving surfaces, any one of which may limit productivity. So, it is important that all receiving surfaces be conducive to coating.
The importance of the surface properties of the receiving surface to coating performance has been largely unrecognized in public technical literature; for example, Buonopane R A, Gutoff E B, & Rinmore MMTR, 1986, AIChE J., 32, p. 682, and Burley, R., 1992, JOCCA., 5, p. 192. However, in formulating and coating photographic products, large differences among receiving surfaces are observed. For example, the limit of coating speed, usually marked by the entrainment of air between the coating compositions and the receiving surface, can vary by an order of magnitude. A receiving surface with poor properties can severely restrict manufacturing performance. Thus, the ability to predict coating performance on various receiving surfaces quickly and inexpensively can be useful and valuable.
Although coating performance cannot be left to chance, methods to predict performance quickly and inexpensively are virtually nonexistent. Past experience with compositions can be helpful, but it is difficult to recognize when a significant change has been made given the chemical complexity of photographic compositions. Predictions based on scientific and engineering models and principles are not possible because much of coating mechanics, and particularly the high speed wetting of a receiving surface that is the heart of coating, remains an enigma (for example, Shikhmurzaev, Y. D., J Fluid Mech., 334, 1997). As a result, in the prior art coating performance is evaluated empirically on pilot coating machines. These coating machines are called pilot machines because they are much narrower than production coating machines, but they must otherwise duplicate manufacturing conditions such as speed. Such pilot machines are therefore expensive to build, operate, and maintain. Thus, the predictive step of pilot coating practiced in prior art is costly and time consuming and at odds with the modern manufacturing objectives already recited. What may be an even worse drawback is that materials being evaluated for new product formulations may not be readily available in the quantities required for high speed pilot coating. So, costly delays in product programs or compromised formulations can result.
One useful method for characterizing the coating latitude of receiving surfaces and maximizing coating speeds based on the results has been disclosed in European Patent Application EP 0 769 717 A1. The method involves measuring the free energy components of receiving surfaces and ensuring through materials selection that these components lie within specified ranges. Specifically, static advancing contact angles of suitably chosen test liquids (water, 1-bromonaphthalene, and 2,2'-thiodiethanol) on the receiving surface are measured, and the method of Fowkes (1962, J. Phys. Chem., 66, p. 382) is employed to determine the free energy components of the receiving surface.
This method for evaluating receiving surfaces for coating speed is useful but has some drawbacks and limitations. The method has proven most reliable for predominantly gelatin/surfactant receiving surfaces, as subbed film is likely to be. Suitable test liquids cannot be found in all cases. The reliable measurement of static advancing contact angles requires considerable skill and experience. Phenomena that occur over the relatively long time of a contact angle measurement but do not occur to a meaningful extent in a coating process can invalidate a measurement. For instance, the test liquid may be adsorbed by the receiving surface, or components of the receiving surface may dissolve or leech into the test liquid, thereby rendering the data questionable. In addition, the time it takes for the test liquid to equilibrate severely limits the capability of the test. So, for some receiving surfaces the method cannot be carried out or the prediction is uncertain.
It would be advantageous to devise a method for evaluating the coatability of substrates for various coating and the appropriate coating temperatures for coating operations for photographic film and paper without having to use a pilot plant operation to determine each situation individually and to avoid the time-consuming process described in European Patent Application 0 769 717 A1.